| STI Engineering |
| Steam Injection |
Enhanced Landfill Gas Production
With Steam
ABSTRACT: The goal of a conventional Landfill Bioreactor is to increase the humidity of the landfill not to
saturate the refuse. Based on several case studies of Landfill Bioreactors involving leachate recirculation
it has been confirmed that increasing the humidity of the landfill will produce as much as seven to ten
times more methane in a shorter period of time than normal MSW landfills with no leachate recirculation.
However it has been reported at LFG Symposiums that only 5% of the water injected is retained and that
95% of the water migrates to the bottom of the landfill. Steam injection will increase the humidity to 100%
with excellent temperature control and will produce the maximum amount of methane based on the
amount of organic material available. Not only will Steam Injection create the maximum amount of LFG but
recover the most airspace in the shortest amount of time. Once leachate/condensate is converted to
steam and is injected into the landfill, it is converted to LFG and extracted, there is no recirculation of the
leachate/condensate.
This paper will outline the advantages of using steam injection versus water addition in landfills. Many of
these advantages are enhanced by the use of the in situ data obtained by the Piezo-Penetrometer Test
(PPT) to properly design and construct the system and evaluate the progress of the Steam Injection in
Landfills. The PPT is also used to install the push-in gas collectors and steam injectors at a fraction of the
cost of drilled in collectors.
This paper will discuss the basic theory of Enhanced LFG Production with Steam Injection based on the
physics of the process involved and then confirm them with the results of a pilot study performed at the
Miramar Landfill in San Diego, California in 2005 and 2006. All of the project goals were achieved.
INTRODUCTION
In August, 2003 STI Engineering presented a proposal to the City of San Diego, Refuse Disposal Division
to perform a Steam Injection Pilot Study at the Miramar Landfill, located in San Diego, California.
On May 5, 2005 STI Engineering received the Right of Entry from the City of San Diego to perform the Steam
Injection Pilot Study.
On May 16, 2005 STI Engineering mobilized a PPT rig and a Geoprobe unit to the subject site.
FIGURE 1
PPT RIG & GEOPROBE
The following outlines the project scope of work, and describes the field operations.
SCOPE OF WORK
The scope of work completed consisted of the following tasks:
- Developed a Health and Safety Plan for the proposed work on the subject site.
- Pre-qualified selected locations on the landfill using the PPT prior to installing 2-inch diameter steel
push-in injectors and collectors. - The PPT data was used to indicate the following:
-Determine the presence of LFG pressure.
-Determine the optimal depths to install the screen sections of the injectors and collectors.
-Verify that no liquid layers were present that could impact the injectors or collectors.
- Installed 6 schedule 80 black steel 2” diameter collectors-extraction wells (EW) with (1/8-inch mill
slot screen) for gas in the test area. - Installed 2 schedule 80 black steel 2” diameter extraction wells (EW) for gas in the control area.
- All extraction wells were connected to a 2” diameter Landtec well heads and then to the vacuum
header. - Installed 3 steam injectors using 2” diameter steel schedule 80 black pipe.
- Installed 9 thermocouples into the refuse at various depths and at various locations in the test area.
Installed 1 thermocouple on each of the 3 steam injectors. - Installed 2 Static Piezometers in the test area.
- Install 5 settlement monuments in the test area and one in the control area.
- Prepared and submitted a progress report.
A Health and Safety Plan was developed prior to the start of the field operations and submitted to the City of
San Diego. A Health & Safety meeting was conducted with all parties involved in the field activities on May
15, prior to the start of the work. A Rae Q-Rae Lower Explosive Limit (LEL) meter was used to monitor the
air inside the PPT rig during operations. The readings never went beyond the background levels at the
landfill site. Operations were conducted in Level D protection.
FIELD OPERATIONS
The field operations began by performing several PPT soundings on a 200-foot grid to determine the
overall conditions of the landfill waste prism. The grid was reduced to 100 feet and then to 50-feet to obtain
enough data in the selected test area to properly design the steam injection and gas collection system.
The PPT was developed in the late 1970s and early 1980s. At the time, the primary purpose of this device
was to measure the pore pressure in the soil to determine the level of the ground water. The PPT cone is
pushed into the landfill by hydraulic rams mounted on a 20-ton truck. The cone is advanced by adding 1-
meter long push rods pre-threaded with a cable connecting the cone to a computer, which digitally records
and displays in real time several parameters continuously during the sounding. The PPT cone measures
tip resistance and sleeve resistance (useful data for soil type, strength and foundation design) the same
as a standard CPT (ASTM 1988). The instrument has inclinometers to ensure that the cone is staying
vertical during advancement through the subsurface or landfill, which adds to the accuracy of the depth
control. The depth control of the sounding is very exact (via a wire-line from an encoder attached to each
advanced push rod segment). Figure 2 presents a typical PPT sounding log and describes how to read the
log.
FIGURE 2
TYPICAL PPT LOG
The following is a brief description of each column displayed on the log:
Column 1 – Depth of sounding in ten foot intervals.
Column 2 – Friction sleeve values as the PPT cone is advanced through the landfill. The values are
presented in tons per square foot (tsf). The friction values are useful in determining a friction ratio, which is
used to identify the type of material the cone is passing through. It is also an indicator of moist conditions
in the landfill.
Column 3 – Tip Resistance or End Bearing (tsf) values indicate the relative density of the material the cone
is penetrating. This value is also used in the friction ratio calculation. The high tip resistance readings can
indicate dense layers or daily cover layers and the low tip values usually indicate refuse.
Column 4 – Pore Pressure values (psi) measure landfill gas pressures, vacuum and liquid head pressure.
On the above PPT log, the Pore Pressure values begin to increase in pressure at a depth of 5 feet below
ground surface (bgs) and continue to increase to about 1 psi of gas pressure down to 60 feet bgs.
Column 5 – Friction ratio (%) is calculated by dividing the friction sleeve value by the tip resistance and is
presented in a percent. In soils, friction ratios of less than 2% typically indicate sandy or gravelly soil
behavior types, while the higher the friction ratio, indicates a more “clay-like” the material (Robertson and
Campanella 1988). Moist municipal solid waste has been found to generally have friction ratios greater
than 2%.
The pore-pressure transducer in the PPT cone can differentiate between hydrostatic head pressure and
gas pressure as indicated in Figure 3 below. This sounding was not performed at the Miramar Site but is
used to clearly demonstrate the capability of the PPT cone. No substantial liquid layers were indicated in
the test area.
FIGURE 3
TYPICAL PPT LOG
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| Copyright © 2010 STI Engineering/ Regis Renaud ALL RIGHTS RESERVED |
The 30 ton PPT rig that was available at the time of
the start date did not have a large enough center
hole to accommodate the 2” diameter pipe used for
the injectors and collectors, therefore a Geoprobe
was used to install the pipes. No drilling was
conducted to install any of the instruments in the
test area, only push-in technology.
the start date did not have a large enough center
hole to accommodate the 2” diameter pipe used for
the injectors and collectors, therefore a Geoprobe
was used to install the pipes. No drilling was
conducted to install any of the instruments in the
test area, only push-in technology.
